Aerogel

What are aerogels?

Aerogels (greek aeros = air) are materials with a stable open porous structure. Unlike hydrogels which are produced in the sol-gel-process their prous structure is filled with air. Usually 95% of an aerogels density is taken up by air. Due to their porous structure aerogels have high specific surface area, efficient heatinsulation and low density. They are stable enough to carry thousandfold their weight.

What are their applications?

Because of their special properties aerogels can serve manifold applications.

The high specific surface area favors their use as support materials in chemical catalysis, drug delivery systems in pharmacy or carrier materials in general. Furthermore they are suitable for absorption or filters. Aerogels are already used in space science as kinetic absorbers within NASA’s Stardust-project.

Aerogels are excellent heat isolators and electrical insulators. In hot air gas particles move faster than in cold air meaning they have higher kinetic energy. Because of winding channels inside the porous structure the particles need to change their direction often and therefore lose their kinetic energy resulting in cooling of the gas. The heat is transferred to the aerogel which then transfers it back to the gas through vibrations.

The low density on the one hand combine with their stability on the other hand make aerogels interesting for lightweight constructions in automotive, air and space travel, where each spared gram has an impact on sustainability.

How to produce aerogels?

Conventionally areogels are produced in the sol-gel-process using silicic acid and its derivates. For start soles are formed. Those soles are silicon oxide particles formed by reaction limited aggregation (RLA). When the soles grew to their desired size the chemical conditions are changed in a way to remove the limiting condition and accelerate the aggregation. The soles then bond with each other resulting in a gelation which follows the principals of diffusion limited aggregation (DLA). A stable and porous structure is formed filled with solvent, a so-called hydrogel. Through indulgent removal of the solvent (usually by super critical drying) the structure remains resulting in aerogels. With carefully choosing the process parameters and abuse of different aggregation mechanics taylormade aerogels can be processed (regarding density, pore size, etc.).

Cellulose aerogels on the other hand are made by dissolving of cellulose and coagulation of the solution in suitable media. Because cellulose already is a polymer no particle growth is required. The Molecules are bound through hydrogen bonds. Control mechanisms for cellulose aggregation are still being researched.

Why use cellulose?

Cellulose is a nearly undepletable resource – a biopolymer usually obtained from wood and plant remains. This provides aerogel synthesis out cost-efficient and renewable feedstock. In addition there are many simple techniques to derivate cellulose, for example hydrophobation. Furthermore crosslinkers can be added to increase the gels stiffness. The main advantage of cellulose aerogels are their elasticity while silica aerogels suffer from the brittleness.

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